Heretofore, as a working vehicle of this sort there has been known a type in which the respective drive shafts are provided for the front wheels and the rear wheels and in which each of the drive shafts is coupled to a transmission via a differential gear, a propeller shaft and a clutch.
There has also been known another type of such a vehicle in which for each wheel a respective hub has a hydraulic motor directly attached thereto so that the wheels may be driven independently of one another.
In the former type of the above mentioned prior art, it is now found that the need for all the wheels, particularly for the front wheels to be coupled in a mechanical construction has made a power transmission device unduly bulky and complicated in construction and further has imposed a limitation in its cost reduction. Also, in such a design, though an engine is mounted on a traveling body, the bulkiness of the power transmission device as mentioned above has made it a hindrance, thus reducing the degree of freedom of the range in which the engine is disposed and resulting in a poor equipment capability about the engine itself. Further, in this conventional technology, owing to the fact the left hand side wheel and the right side wheel on the front side are designed to be driven with a common drive shaft, it have been impossible for a difference to be given between the left hand side and right hand side with respect to the rate of rotation and the torque thereof, so that only a poor ability for the vehicle to travel may then ensue on a soft and weak site and on an unleveled ground.
On the other hand, in the latter type of the above mentioned prior art, it has now been found that the structure in which all the wheels are driven in a hydraulic mechanism makes it inevitable for its traveling efficiency to be lowered because of those characteristics that are inherent in any hydraulic motor. Also, in such a design in which the hydraulic circuit used for the vehicle to travel is naturally constructed of a closed circuit, it has been made effectively impractical for this hydraulic circuit to be coupled to a hydraulic circuit for a working machine and, as a result, the requirement for a respective hydraulic pump as for each of a hydraulic circuit for the vehicle body to travel and a hydraulic circuit for the working machine to be operated has made the entire hydraulic arrangement quite large scaled.
It should also be noted at this point that in the former type of the above mentioned prior art, even if the input shaft of the transmission is arranged so as to be driven by a hydraulic motor, it is found that since such a hydraulic motor has to be driven with a closed circuit, in this design, too, a hydraulic circuit for the vehicle body to travel and a hydraulic circuit for the working machine to be operated have had to be provided in their whole construction separately entirely from each other.
The present invention has been made to obviate the above mentioned inconveniences in the prior art and has for its object to provide a traveling drive apparatus in a working vehicle, which is made capable of effecting the simplification of a power transmission device therein, capable of reducing the speed ratios of the vehicle in a transmission therein, capable of reducing the cost involved in the said power transmission device as well as in the said transmission, capable of increasing the degree of freedom of the range in which an engine mounted at the lower side of a traveling body for the said vehicle is disposed, capable of enhancing the ability of the said vehicle to travel on a soft and weak ground site as well as on an unleveled or unprepared ground, capable of effecting the simplification of a hydraulic circuit arrangement in its construction and further capable of protecting, from an obstacle on the road on which the vehicle is traveling, hydraulic motors provided for independently driving the vehicle wheels.